Enhanced Differential Methods for Guided Wave Phased Array Imaging Using Spatially Distributed Piezoelectric Transducers
10 Mar 2006-Quantitative Nondestructive Evaluation (American Institute of Physics)-Vol. 820, Iss: 1, pp 837-844
TL;DR: In this article, a number of tomographic and phased array methods have been proposed for generating two dimensional images of plate-like structures using sparse arrays of spatially distributed ultrasonic transducers.
Abstract: A number of tomographic and phased‐array methods have been proposed for generating two dimensional images of plate‐like structures using sparse arrays of spatially distributed ultrasonic transducers. The phased array differential approach is considered here whereby pulse echo and through transmission signals are recorded before and after localized damaged is introduced, and differenced signals are combined using a focusing rule to produce an image of the plate. The application is structural health monitoring where the transducers are permanently bonded to the structure. The quality of the image is affected by many factors such as the number and location of the transducers, the characteristics of the damage, the signal‐to‐noise ratio, presence of edge reflections, and anything unrelated to damage that may perturb the ultrasonic signals such as temperature changes and transducer bonding variations. Two methods for enhancing image quality are implemented and then evaluated as to their effectiveness. In the first method, the windowing function is changed in width prior to phased signal addition to yield the best image quality. In the second method, signals are envelope‐detected prior to phased signal addition to eliminate phasing artifacts. Results are reported for artificial defects introduced in aluminum plates.
Citations
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TL;DR: In this article, the authors proposed a two-step process whereby damage is first detected and is then localized and characterized, based on the early time regime when discrete echoes from boundary reflections and scattering sites are meaningful.
Abstract: Permanently attached piezoelectric sensors arranged in a spatially distributed array are under consideration for structural health monitoring systems incorporating active ultrasonic methods. Most damage detection and localization methods that have been proposed are based upon comparing monitored signals to baselines recorded from the structure prior to initiation of damage. To be effective, this comparison process must take into account any conditions other than damage that have changed the ultrasonic signals. Proposed here is a two-step process whereby damage is first detected and is then localized and characterized. The detection strategy considers the long time behavior of the signals in the diffuse-like regime where distinct echoes can no longer be identified. The localization strategy is to generate images of damage based upon the early time regime when discrete echoes from boundary reflections and scattering sites are meaningful. Results are shown for an aluminum plate with artificial damage introduced in combination with temperature variations. The loss of local temporal coherence combined with an optimal baseline selection procedure is shown to be effective for the detection of damage, and a delay-and-sum imaging method applied to the residual signals both localizes the damage and provides characterization information.
449 citations
Cites methods from "Enhanced Differential Methods for G..."
...Michaels and Michaels [15, 16] have applied this method to both raw and envelope-detected residual signals to generate localization images of artificial damage in aluminum plates....
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...Michaels et al [12] similarly construct ellipses, but localize damage via the sum of the distances to all of the ellipses calculated over the area of interest....
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...Lu and Michaels [9] also consider a differential feature approach, and apply data fusion methods to improve the probability of detection while maintaining a low false alarm rate....
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...Michaels et al [8] consider a set of time and frequency domain differential features and evaluate various combinations for damage detection....
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TL;DR: In this article, a spatially distributed array of single element piezoelectric transducers is considered for monitoring the integrity of plate-like structures over large areas, and time shift averaging algorithms are applied to differential signals filtered at multiple frequencies, resulting in many images for the same structural state.
297 citations
TL;DR: In this article, a structural health monitoring system based on the excitation and reception of guided waves using piezoelectric elements as sensors is described, and the baseline subtraction approach is used to detect defects in a simple rectangular plate.
Abstract: It is desirable for any structural health monitoring (SHM) system to have maximum sensitivity with minimum sensor density. The structural health monitoring system described here is based on the excitation and reception of guided waves using piezoelectric elements as sensors. One of the main challenges faced is that in all but the most simple structures the wave interactions become too complex for the time domain signals to be interpreted directly. One approach to overcoming this complexity is to subtract a baseline reference signal from the measured system when it is known to be defect free. This strategy enables changes in the structure to be identified. Two key issues must be addressed to allow this paradigm to become a reality. First, the system must be sufficiently sensitive to small reflections from defects such as cracking. Second, it must be able to distinguish between benign changes and those due to structural defects. In this paper the baseline subtraction approach is used to detect defects in a simple rectangular plate. The system is shown to work well in the short term, and good sensitivity to defects is demonstrated. The performance degrades over the medium to long term. The principal reason for this degradation is shown to be the effect of change in temperature of the system. These effects are quantified and strategies for overcoming them are discussed.
240 citations
TL;DR: Utilization of MVDR significantly improves image quality compared with delay-and-sum imaging, and additional improvements are obtained from incorporation of a priori scattering information in the MVDR method, use of phase information, and instantaneous windowing.
Abstract: Ultrasonic guided wave imaging with a sparse, or spatially distributed, array can detect and localize damage over large areas. Conventional delay-and-sum images from such an array typically have a relatively high noise floor, however, and contain artifacts that often cannot be discriminated from damage. Considered here is minimum variance distortionless response (MVDR) imaging, which is a variation of delay-and-sum imaging whereby weighting coefficients are adaptively computed at each pixel location. Utilization of MVDR significantly improves image quality compared with delay-and-sum imaging, and additional improvements are obtained from incorporation of a priori scattering information in the MVDR method, use of phase information, and instantaneous windowing. Simulated data from a through-hole scatterer are used to illustrate performance improvements, and a performance metric is proposed that allows for quantitative comparisons of images from a known scatterer. Experimental results from a through-hole scatterer are also provided that illustrate imaging efficacy.
166 citations
Cites background from "Enhanced Differential Methods for G..."
...The envelope is obtained by taking the absolute value of the analytic representation of the scattered signal: r t s t j tsij ij ijENV( ) ( ) ( ) ,ˆ= + (1) where sij(t) is the rF scattered signal generated by transmitting from the ith transducer and receiving at the jth transducer, ˆ ( )s tij is the Hilbert transform of the rF scattered field, j is the square root of −1, and r tijENV( ) is the envelope signal. combining the imaging concepts of Wang et al. [17] and Michaels and Michaels [18], pixel values for delayand-sum imaging are defined as: P e r t d d c w t tx y ijxy ij ixy jxy j i N i N , ( )= + +æ è ççç ö ø ÷÷÷ = += - åå g d 11 1 2 ò , (2) where w(t) is a windowing function, dixy is the distance from the ith transducer to the pixel location (x, y), cg is the group velocity, and eijxy is a weighting coefficient specific to the pixel location and transmitter-receiver pair....
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...[17] and Michaels and Michaels [18], pixel values for delayand-sum imaging are defined as:...
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...Finally, Michaels and Michaels [20] showed that images generated from different frequency excitation signals could be fused to obtain higher-quality images. another technique has recently been introduced, the hyperbola algorithm [19], [21], which applies the delayand-sum approach to cross-correlations between baselinesubtracted signals....
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...Michaels and Michaels [18] expanded the approach of Wang et al. to sum the signals over a time window of data, as opposed to a single point in time, establishing the pixel value as the energy contained within the resulting summed signal....
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...Michaels and Michaels [18] expanded the approach of Wang et al....
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16 Apr 2007
TL;DR: In this article, the authors investigate the suitability and robustness of OBS under varying rates of temperature change and its use in the localization of damage in a structural health monitoring (SHM) system.
Abstract: When testing complex structures using a deployable guided wave system, the measured signals are often far too complex to directly interpret without a priori knowledge of the geometry of the structure. However, in a structural health monitoring (SHM) scenario, the sensors are permanently attached to the structure and so changes in the system can be monitored by comparison to an earlier baseline measurement, taken when the structure was undamaged. This paper investigates such a system for the SHM of plate-like structures using guided waves. The basis of the system is a distributed array of permanently attached piezoelectric guided wave transducers where pairs of transducers are used in pitch-catch configuration allowing the detection and localization of damage in the plate. It has been observed that the use of a single baseline measurement suffers in the medium to long-term due to the variation of environmental conditions. A key parameter in the instability is the change in the temperature of the test structure. A method of using a database of baselines, termed optimal baseline subtraction (OBS), is described and applied to an experimental SHM system. The objective of this paper is to investigate the suitability and robustness of OBS under varying rates of temperature change and its use in the localization of damage. Results have shown that through the OBS an improvement of up to 20 dB in signal-to-coherent noise ratio may be achieved compared with single baseline subtraction
122 citations
Cites methods from "Enhanced Differential Methods for G..."
...This method is similar to those presented in [9], [27], and [28]....
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References
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Book•
01 Sep 2004
TL;DR: In this article, the theory of elasticity was introduced and basic formulas and concepts in complex variables in the theory and application of wave propagation were discussed. But the authors did not consider the effects of wave scattering on the wave propagation experiments.
Abstract: Preface 1. Introduction 2. Dispersion principles 3. Unbounded isotropic and anisotropic media 4. Reflection and refraction 5. Oblique incidence 6. Wave scattering 7. Surface and subsurface waves 8. Waves in plates 9. Interface waves 10. Layer on a half space 11. Waves in rods 12. Waves in hollow cylinders 13. Guided waves in multiple layers 14. Source influence 15. Horizontal shear 16. Waves in an anisotropic layer 17. Elastic constant determination 18. Waves in viscoelastic media 19. Stress influence 20. Boundary element methods Bibliography Appendices A. Ultrasonic nondestructive testing principles, analysis and display technology B. Basic formulas and concepts in the theory of elasticity C. Basic formulas in complex variables D. Schlieren imaging and dynamic photoelasticity E. Key wave propagation experiments Index.
2,570 citations
"Enhanced Differential Methods for G..." refers background in this paper
...17 MHz-mm, indicating that this mode should exhibit dispersion [10]....
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...56 mm/μs, which corresponds to the So Lamb wave mode in the plate [10]....
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TL;DR: In this article, the applicability of the time-reversal concept to guided waves in plate-like structures, where the stress waves are dispersive and of multi-modes, was investigated.
Abstract: This paper presents an experimental and theoretical investigation of the applicability of the time-reversal concept to guided waves in plate-like structures, where the stress waves are dispersive and of multi-modes. It is shown that temporal and spatial focusing can be achieved through time reversal, although the dispersive behaviour of the flexural waves renders it impossible to exactly reconstruct the waveform of the original excitation. Based on the principle of the time-reversal concept, a digital imaging method suitable for distributed sensor/actuator networks has been developed. This new method, which overcomes the limitation of the conventional phased array method that operates under pulse-echo mode, provides an efficient imaging method for locating and approximate sizing of structural damages. In addition, it has been shown that signal strengths can be considerably enhanced by applying the present synthetic time-reversal method, thus reducing the number of sensors and actuators required to achieve a given signal-to-noise ratio.
504 citations
"Enhanced Differential Methods for G..." refers background in this paper
...[9]; here two enhancements are described and results presented....
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...Many damage detection algorithms using fixed transducers rely on monitoring changes since the coupling and transducer variations common for nondestructive testing are no longer present [3,7,8,9]....
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TL;DR: A methodology for applying diffuse ultrasonic waves to the problem of detecting structural damage in the presence of unmeasured temperature changes and it is shown that a probability of detection of over 95% can be achieved with a small number of baseline waveforms.
353 citations
"Enhanced Differential Methods for G..." refers background in this paper
...Many damage detection algorithms using fixed transducers rely on monitoring changes since the coupling and transducer variations common for nondestructive testing are no longer present [3,7,8,9]....
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TL;DR: Experimental data obtained from a guided wave array containing electromagnetic acoustic transducers (EMAT) elements for exciting and detecting the S/sub 0/ Lamb wave mode in a 5-mm thick aluminium plate are processed with both algorithms and the results are discussed.
Abstract: Omni-directional guided wave array transducers contain a circular pattern of elements that individually behave as omni-directional point transmitters or receivers. The data set acquired from such an array contains time-domain signals from each permutation of transmitter and receiver. A phased addition algorithm is developed that allows an omni-directional, B-scan image of the surrounding plate to be synthesized from any geometry of array. Numerically simulated data from a single reflector is used to test the performance of the algorithm. The results from an array containing a fully populated circular area of elements (Type I array) are found to be good, but those from an array containing a single ring of elements (Type II array) contain many large side-lobes. An enhancement to the basic phased addition algorithm is presented that uses deconvolution to suppress these side-lobes. The deconvolution algorithm enables a Type II array to equal the performance of a Type I array of the same overall diameter. The effect of diameter on angular resolution is investigated. Experimental data obtained from a guided wave array containing electromagnetic acoustic transducers (EMAT) elements for exciting and detecting the S/sub 0/ Lamb wave mode in a 5-mm thick aluminium plate are processed with both algorithms and the results are discussed.
269 citations
"Enhanced Differential Methods for G..." refers methods in this paper
...Techniques used have been tomographic reconstruction [3,4] and compact phased array imaging [4,5,6]....
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Journal Article•
TL;DR: In this paper, the authors focus on long range testing using frequencies below 100 kHz and describe the progress from research work to a robust, commercial pipe testing system, together with the more recent development of a test for rail.
Abstract: The testing of large structures using conventional ultrasonic bulk wave techniques is slow because the test region is limited to the area immediately surrounding the transducer. Therefore, scanning is required if the whole structure is to be tested. Ultrasonic guided waves potentially provide an attractive solution to this problem because they can be excited at one location on the structure and will propagate many meters, returning echoes indicating the presence of corrosion or other discontinuities. However, guided wave testing is complicated by the presence of many possible wave modes, most of which are dispersive. These guided wave characteristics offer a wealth of opportunities for the extraction of information about the structure, but it is crucial to manage this complexity if the test is to be useable in industrial practice. Guided waves can be used in three regimes, which have been researched extensively by many authors: short range (« 1 m [39 in.], for example leaky lamb wave testing of composite materials and high frequency surface wave scanning), medium range (up to about 5 m [16.4 ft], for example shear horizontal and lamb waves in the 250 kHz to 1 MHz frequency range for plate and tube testing) and long range (up to 100 m [328 ft], for example the testing of pipelines). This paper concentrates on long range testing using frequencies below 100 kHz. The progress from research work to a robust, commercial pipe testing system is described, together with the more recent development of a test for rail. Future directions for guided wave testing are then discussed.
252 citations
"Enhanced Differential Methods for G..." refers background in this paper
...Recent work has shown much promise in terms of detecting, locating and characterizing defects in both plates and pipes [1,2]....
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